Method and apparatus for measuring oxidation-reduction potential

ABSTRACT

Methods and systems for measuring the oxidation-reduction potential of a fluid sample are provided. The system includes a test strip with a sample chamber adapted to receive a fluid sample. The sample chamber can be associated with a filter membrane. The test strip also includes a reference cell. The oxidation-reduction potential of a fluid sample placed in the sample chamber can be read by a readout device interconnected to a test lead that is in electrical contact with the sample chamber, and a reference lead that is in electrical contact with the reference cell. Electrical contact between a fluid sample placed in the sample chamber and the reference cell can be established by a bridge. The oxidation-reduction potential may be read as an electrical potential between the test lead and the reference lead of the test strip.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/447,568, filed Feb. 28, 2011, the entiredisclosure of which is hereby incorporated herein by reference.

FIELD

The present invention relates to methods and apparatuses for measuringthe oxidation-reduction potential of a fluid sample.

BACKGROUND

Whole blood and blood products, such as plasma and serum, haveoxidation-reduction potentials (ORP). Clinically the ORP of blood,plasma and serum provides a diagnostic assay of the oxidative status ofan animal. More particularly, researchers have determined that the ORPof blood, plasma and serum is related to health and disease.

An oxidation-reduction system, or redox system, involves the transfer ofelectrons from a reductant to an oxidant according to the followingequation:oxidant+ne ⁻⇄reductant  (1)where ne⁻ equals the number of electrons transferred. At equilibrium,the redox potential (E), or oxidation-reduction potential (ORP), iscalculated according to the Nernst-Peters equation:E(ORP)=E _(o) −RT/nF ln [reductant]/[oxidant]  (2)where R (gas constant), T (temperature in degrees Kelvin) and F (Faradayconstant) are constants. E_(o) is the standard potential of a redoxsystem measured with respect to a hydrogen electrode, which isarbitrarily assigned an E_(o) of 0 volts, and n is the number ofelectrons transferred. Therefore, ORP is dependent on the totalconcentrations of reductants and oxidants, and ORP is an integratedmeasure of the balance between total oxidants and reductants in aparticular system. As such, ORP provides a measure of the overalloxidative status of a body fluid or tissue of a patient.

An ORP measurement which is significantly higher than that of normalswill indicate the presence of oxidative stress. Oxidative stress hasbeen related to many diseases, and it has been found to occur in alltypes of critical illnesses. Accordingly, an ORP level significantlyhigher than that of normals indicates the presence of a disease andperhaps a critical illness. An ORP measurement which is the same as orlower than that of normals indicates the absence of oxidative stress andthe absence of a disease or critical illness. Thus, the ORP level of apatient can be used by a medical doctor or veterinarian as an aid indiagnosing or ruling out the presence of a disease, particularly aserious illness. Sequential measurements of ORP over time can be used tomonitor the progression of a disease and the effectiveness or lack ofeffectiveness of treatment of the disease. If a patient's ORP does notdecrease after treatment, or especially if it increases despitetreatment, this may indicate a poor prognosis and the need for moreaggressive and/or additional and/or different treatments. In the case ofa measurement made by a patient, such as a patient experiencing symptomsof myocardial infarction, the ORP level may indicate the need for thepatient to see a doctor or to immediately proceed to an emergency roomfor treatment.

Oxidative stress is caused by a higher production of reactive oxygen andreactive nitrogen species or a decrease in endogenous protectiveantioxidative capacity. Oxidative stress has been related to variousdiseases and aging, and it has been found to occur in all types ofcritical illnesses. See, e.g., Veglia et al., Biomarkers, 11(6): 562-573(2006); Roth et al., Current Opinion in Clinical Nutrition and MetabolicCare, 7:161-168 (2004); U.S. Pat. No. 5,290,519 and U.S. PatentPublication No. 2005/0142613. Several investigations have shown a closeassociation between the oxidative status of a critically ill patient andthe patient's outcome. See Roth et al., Current Opinion in ClinicalNutrition and Metabolic Care, 7:161-168 (2004).

Oxidative stress in patients has been evaluated by measuring variousindividual markers. See, e.g., Veglia et al., Biomarkers, 11(6): 562-573(2006); Roth et al., Current Opinion in Clinical Nutrition and MetabolicCare, 7:161-168 (2004); U.S. Pat. No. 5,290,519 and U.S. PatentPublication No. 2005/0142613. However, such measurements are oftenunreliable and provide conflicting and variable measurements of theoxidative status of a patient. See Veglia et al., Biomarkers, 11(6):562-573 (2006); Roth et al., Current Opinion in Clinical Nutrition andMetabolic Care, 7:161-168 (2004). The measurement of multiple markerswhich are then used to provide a score or other assessment of theoverall oxidative status of a patient has been developed to overcome theproblems of using measurements of single markers. See Veglia et al.,Biomarkers, 11(6): 562-573 (2006); Roth et al., Current Opinion inClinical Nutrition and Metabolic Care, 7:161-168 (2004). Although suchapproaches are more reliable and sensitive than measurements of a singlemarker, they are complex and time consuming. Thus, there is a need for asimpler and faster method for reliably measuring the overall oxidativestatus of a patient.

The oxidation/reduction potential can be measured electrochemically.Electrochemical devices for measuring ORP of blood and blood productstypically require large sample volumes (that is, ten to hundreds ofmilliliters) and long equilibrium periods. Furthermore, theelectrochemical devices have large, bulky electrodes that requirecleaning between sample measurements. Such electrochemical devices arepoorly suited for routine clinical diagnostic testing. It has beensuggested to use electrodes that have undergone treatment to preventbiofouling. However, such devices necessarily involve complexmanufacturing techniques. Moreover, conventional electrochemical deviceshave not provided a format that is convenient for use in a clinicalsetting.

The oxidative and radical characteristics of human blood plasma and itsblood components (such as low density lipoproteins, serum albumin, andamino acids) can also be determined from photo chemiluminescence, withand without thermo-initiated free radical generation. A photochemiluminescent system generally includes a free radical generator anda detector that measures chemiluminometric changes in the presence of anantioxidant. More specifically, the blood plasma sample (or one of itscomponents) containing an amount of antioxidant is contacted and reactedwith a known amount of free radicals. The free radicals remaining aftercontacting the blood plasma sample are determined chemiluminometrically.This type of measurement and detection system is not suitable for rapid,large scale measurements of blood plasma samples in a clinical setting.

SUMMARY

Embodiments of the present invention are directed to solving these andother problems and disadvantages of the prior art, and provide systemsand methods for measuring oxidation-reduction potential (ORP) that aresuitable for rapid, routine clinical diagnostic testing. The systemgenerally includes a test strip and a readout device. More particularly,embodiments of the present invention system can determine the ORP of abody fluid of a patient, including blood, plasma and serum, or a fluidfrom an in vitro source, such as, but not limited to extracellular andintracellular fluids (as for example, aqueous humour, vitreous humour,breast milk, cerebrospinal fluid, cerumen, endolymph, perilymph, gastricjuice, mucus, peritoneal fluid, pleural fluid, salvia, sebum, semen,sweat, tears, vaginal secretion, vomit, and urine).

The test strip generally includes a substrate, one or more test leads, areference lead, a reference cell, and a bridge. In a preferredembodiment, the one or more test leads, the reference lead, thereference cell and the bridge are located between an overlay and thesubstrate. A sample chamber generally encompasses at least a portion ofthe bridge and a portion of each of the one or more test leads. The oneor more test leads may comprise a working electrode and a counterelectrode. In one embodiment, a sample region comprising the samplechamber is defined by an aperture, the aperture being contained withinthe overlay. Alternatively or in addition, the sample chamber includes adepression or well within the substrate, or an aperture or well in anintermediate layer. The sample chamber is generally configured tocontain a fluid sample, such as blood and/or a blood product. The fluidsample generally comprises a volume of less than about 1 ml. Preferably,the volume of the fluid sample is about a drop of blood (e.g., 0.05 ml)or less. In accordance with embodiments of the present invention, thebridge is wetted by the fluid sample, to place the bridge and at leastportions of the sample chamber in electrical contact with the referencecell.

The substrate can comprise a dielectric material and may have asubstantially planar surface. In accordance with embodiments of thepresent invention, the overlay may comprise a dielectric material. Theoverlay may be bonded or laminated to the substrate.

The leads generally comprise an electrically conductive material havinga substantially continuous and/or uniform composition. Moreparticularly, the leads may comprise a noble metal or other electricallyconductive material. As an example, the leads may comprise anelectrically conductive ink that is deposited on the substrate in aprinting process. The one or more test leads generally extend from thesample chamber to a readout region, and the reference lead generallyextends from the reference cell to the readout region. The readoutregion contains electrical contacts associated with the leads, and isgenerally adapted to operatively interconnect to the readout device andto form an electrical contact between the readout device and at leastone test lead and the reference lead.

The reference cell generally provides a known voltage potential. Withoutlimitation, the reference cell can comprise one of a silver/silverchloride half-cell, a copper/copper sulfate half-cell, amercury/mercurous chloride half-cell, and a standard hydrogen half-cell.

The bridge is provided to establish electrical contact between a fluidsample in the sample chamber and the reference cell. The bridge caninclude an electrolytic solution, an ionic gel, a filter, or any waterwicking or water transporting material, such as paper. The bridge isgenerally positioned between the sample chamber and the reference cell.

In practice, electrical contact is established between the leads when asuitable fluid sample is placed in the sample chamber, and the bridge isoperative to place the fluid sample and the reference cell in electricalcontact with one another. For example, where the bridge comprises awater transporting material, the bridge is operative to establishelectrical contact between the fluid sample and the reference cell whenthe bridge is sufficiently wetted to establish an electrical contactwith the reference cell and the fluid sample. Furthermore, an electricalcircuit is established when a fluid sample is placed in the samplechamber 120 and two or more of the leads are operatively interconnectedto the readout device.

The readout device generally comprises a voltmeter, galvanostat,potentiostat or other device that is capable of reading a potentialdifference comprising or representative of the ORP of the fluid sampleby electrically interconnecting to the working electrode, the counterelectrode, and/or the reference lead of the test strip. Examples ofsuitable readout devices include, without limitation, analog voltmeters,digital voltmeters, analog null-balance voltmeters, galvanostats, andpotentiostats. In some embodiments, the readout device can have aprocessor that includes and/or is associated with a memory forcontrolling one or more optional aspects of the readout device. Withoutlimitation, the processor can execute instructions stored in memory, canimplement a process according to measured voltage, and/or can implementa process according to a time interval. The readout device can furtherinclude one or both of a user input and a user output. Examples of theuser output include, without limitation, one or more of a digital outputthat displays an oxidation/reduction potential value, indicator lamp(s),machine generated speech, and an audible tone sequence. Examples of theuser input include, without limitation, buttons, switches, a keypad, akeyboard, and/or a touch screen interface for receiving input from theuser. The user input can receive input to control one or more of inputto: power on or power off the readout device, perform diagnosticsrelated to the proper operation of the readout device, receive inputrelated to various operational parameters or control other operations orfunctions.

Another aspect of the present invention is a method of using the systemto determine the ORP of a sample. The method generally includes thefollowing steps: a) obtaining a fluid sample; b) placing the fluidsample in the sample chamber of the test strip; c) using a bridge tosubstantially establish electrical contact between the sample chamberthe reference cell; d) interconnecting a test electrode and a referenceelectrode of the test strip to a readout device; e) determining the ORPafter a selected interval. In one configuration, step b) furtherincludes separating a plasma component from a whole blood fluid sample,wherein the plasma is collected in the sample chamber. In anotherconfiguration, step d) further includes interconnecting a counterelectrode to the readout device, passing a current between the workingelectrode and the counter electrode, and reading a voltage potentialbetween the reference electrode and the working electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a system for measuring the oxidation-reduction potentialof a fluid in accordance with embodiments of the present invention;

FIG. 2 illustrates components of a test strip in accordance withembodiments of the present invention;

FIG. 3 depicts a test strip overlay component in accordance withembodiments of the present invention;

FIG. 4 illustrates the relationship of components in an assembled teststrip in accordance with embodiments of the present invention;

FIG. 5 illustrates a test strip in accordance with other embodiments ofthe present invention in plan view;

FIG. 6 is a cross-section of the test strip illustrated in FIG. 5, takenalong section line A-A;

FIG. 7 is a partial cross-section of the test strip illustrated in FIG.6, taken from within detail area B;

FIG. 8 is an exploded view of the test strip illustrated in FIG. 5;

FIG. 9 is a top plan view of the substrate of the test strip shown inFIG. 5 in accordance with embodiments of the present invention;

FIG. 10 is a bottom plan view of the test strip substrate shown in FIG.5 in accordance with embodiments of the present invention;

FIG. 11 is a view of the test strip substrate shown in FIG. 5 inelevation in accordance with embodiments of the present invention;

FIG. 12 is an exploded view of a test strip in accordance with furtherembodiments of the present invention;

FIG. 13 is a block diagram depicting components of a readout device inaccordance with embodiments of the present invention;

FIG. 14 is a flow chart depicting aspects of a process for measuring theoxidation-reduction potential of a fluid sample in accordance withembodiments of the present invention;

FIG. 15 is an exploded elevation view of a test strip in accordance withother embodiments of the present invention;

FIG. 16 is a top plan view of the test strip according to FIG. 15;

FIG. 17 is a top plan view of a test strip in accordance with furtherembodiments of the present invention;

FIG. 18 depicts components of readout electronics 1304 and aninterconnected test strip 104 in accordance with embodiments of thepresent invention;

FIG. 19 is a flowchart depicting aspects of a process for measuring theoxidation-reduction potential of a fluid sample in accordance with otherembodiments of the present invention; and

FIGS. 20A-B are graphs depicting exemplary ORP values for normal andtrauma plasma using a test strip and readout apparatus in accordancewith embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 depicts a system 100 for measuring the oxidation-reductionpotential of a fluid sample in accordance with embodiments of thepresent invention. The system 100 generally includes a test strip 104and a readout device 108. Also shown as a part of the system 100 is afluid sample source 112 for supplying a fluid sample 116.

The test strip 104 generally includes a sample chamber 120. The samplechamber 120 may correspond to a test strip overlay aperture 124 formedin a test strip overlay 128. The test strip overlay 128 may beinterconnected to a test strip substrate 132. A number of electricalcontacts 136 may be provided in a readout region 140. The electricalcontacts 136 may be associated with various leads and other componentsof the test strip 104, as will be described in greater detail elsewhereherein.

The readout device 108 may include a set of readout device contacts 144.The readout device contacts 144 are generally configured to establish anelectrical connection between the readout device 108 and the electricalcontacts 136 of the test strip 104. As shown in the example system 100,the readout device contacts 144 may be associated with a readoutaperture 148 that receives the readout region 140 of the test strip 104when the test strip 104 is joined with the readout device 108 such thatan electrical signal can be read from the electrical contacts 136 of thetest strip 104 by the readout device 108. Alternatively, the readoutdevice contacts 144 may comprise two or more flexible wires or leadsthat can be brought into contact with the electrical contacts 136 of thetest strip 104.

In general, the readout device 108 comprises a voltmeter. Moreparticularly, the readout device 108 operates to read a voltage betweentwo readout contacts. Accordingly, the readout device contacts 144operate to read an electrical potential or a voltage between any two ofthe electrical contacts 136 of the test strip 104. In accordance withfurther embodiments, the readout device 108 may perform a galvanostaticmeasurement, as described in greater detail elsewhere herein.Alternatively, in accordance with embodiments of the present invention,rather than providing three electrical contacts 136, a test strip 104can include two electrical contacts 136. Similarly, the readout device108 can include two readout device contacts 144. Moreover, theparticular arrangement of readout device contacts 144 and/or readoutaperture 148 can vary in order to accommodate different electricalcontact 136 and readout region 140 arrangements of different test strips104.

The readout device 108 may additionally include a user output 152. Forexample, the user output 152 can comprise a visual display for providingoxidation-reduction potential information regarding the fluid sample 116to a practitioner. Alternatively or in addition, the user output 152 cancomprise a speaker or other source of audible output. In addition, auser input 156 may be provided to allow a practitioner to controlaspects of the operation of the readout device 108.

In accordance with embodiments of the present invention, the fluidsample 116 may comprise blood or a blood product. For example, the fluidsample 116 can include human whole blood or plasma. The fluid samplesource 112 can comprise any vessel or apparatus suitable for placing anappropriate volume of sample fluid 116 in the sample chamber 120 of thetest strip 104. Accordingly, examples of a sample fluid apparatus 112include a syringe, a lancet, a pipette, a vial or other vessel ordevice.

FIG. 2 illustrates components of a test strip 104 with the test stripoverlay 128 removed. In general, the substrate 132 carries and/or hasformed thereon a number of electrically conductive leads 204 thatterminate in the test strip readout contacts 136. The substrate 132itself may comprise a dielectric material. Moreover, the substrate 132may comprise a substantially planar surface on which various componentsof the test strip 104 may be interconnected or formed. In accordancewith further embodiments, the test strip 104 substrate 132 may comprisea depression or well 206 in an area corresponding to the sample chamber120 of the test strip 104.

At least one of the leads 204 is a first test lead or working electrode208 that extends between a first area 212 corresponding to or within thesample chamber 120 of the test strip 104 and a second area 216corresponding to the readout contact 136 of the working electrode 208.In accordance with embodiments of the present invention, at least thefirst area 212 of the working electrode 208 is formed from anelectrically conductive material having a substantially continuousand/or uniform composition. It should be understood that, as usedherein, a substantially continuous and/or uniform composition means thatthe material comprising the working electrode 208 has the same chemicalcomposition and/or a molecular structure at any point in a cross sectionof a portion of the working electrode 208 as at any other point in thecross section of the working electrode 208. More particularly, theelectrically conductive material of the working electrode 208 ispreferably not coated or substantially not coated by a substanceselected to chemically interact with respect to the sample fluid 116.

As examples, and without necessarily importing limitations into theclaims, the working electrode 208 may comprise an electricallyconductive ink deposited on the substrate 132 in a printing operation.In accordance with further exemplary embodiments, the working electrode208 may comprise an electrically conductive layer laminated or otherwisejoined to the substrate 132.

A test strip 104 in accordance with embodiments of the present inventionadditionally includes a lead 204 comprising a reference lead orelectrode 220. The reference lead 220 generally extends between areference cell 224 and a readout region of the reference lead 228. Inaccordance with exemplary embodiments of the present invention, thereference lead 220 may be formed using the same or similar process asthe working electrode.

The reference cell 224 is selected to provide a known voltage potential.For example, the reference cell 224 may comprise a silver/silverchloride, copper/copper sulfate, mercury/mercurous chloride, standardhydrogen electrode, or other electrochemical reference half-cell.

A bridge 232 extends between the reference cell 224 and the samplechamber 120. In accordance with embodiments of the present invention,the bridge 232 may comprise a filter. For example, the bridge 232 may beformed from filter paper. As can be appreciated by one of skill in theart after consideration of the present disclosure, when a fluid sample116 is placed in the sample chamber 120, the filter paper is wetted,establishing an electrically conductive bridge 232 between the fluidsample 116 in the sample chamber 120 and the reference cell 224.

A test strip 104 in accordance with embodiments of the present inventionmay also include a second test lead or counter electrode 236. Thecounter electrode 236 may generally mirror the working electrode 208.Accordingly, the counter electrode 236 may be formed from asubstantially continuous or uniform electrically conductive substancethat extends from a first area 240 that is coincident with the samplechamber 120, to a second area 244 corresponding to the readout portion136 of the counter electrode 236.

With reference now to FIG. 3, a test strip 104 overlay 128 in accordancewith embodiments of the present invention is illustrated in plan view.The test strip 104 overlay 128 includes a test strip aperture 124corresponding to the sample chamber 120 of the assembled test strip 104.In accordance with embodiments of the present invention, the test stripoverlay 128 may comprise a planar piece of dielectric material that isbonded or laminated to the substrate 132 such that the leads 204,reference cell 224, and bridge 232 are held between the substrate 132and the overlay 128. In accordance with further embodiments of thepresent invention, a filter or filter element 304 may extend across thetest strip aperture 124. The filter 304 may comprise a membrane thatfunctions to allow plasma in a fluid sample 116 comprising whole bloodto pass through the test strip aperture to the sample chamber 120. Inaccordance with at least some embodiments of the present invention, thefilter 304 may comprise filter paper. Moreover, in accordance with otherembodiments of the present invention, the filter 304 may extend betweenthe sample chamber 120 and the reference cell 224 to form a bridge 232at least when the filter 304 is wetted.

FIG. 4 illustrates an assembled test strip 104 in accordance withembodiments of the present invention in plan view. Moreover, variousfeatures of the test strip 104 that are under the test strip overlay 128in the assembled test strip 104 are show by dotted lines, to illustratetheir relative locations. As can be appreciated by one of skill in theart after consideration of the present disclosure, absent the presenceof a suitable fluid sample 116 in the sample chamber 120, the variousleads 204 are not in electrical contact with one another. In particular,electrical contact between the leads 204 is not established until asuitable fluid sample 116 is placed in the sample chamber 120, and thebridge 232 has been sufficiently wetted to place the reference lead 220into electrical contact with the working electrode 208 and/or thecounter electrode 236 through the fluid sample 116. Moreover, anelectrical circuit including any two of the leads 204 is not completeduntil the test strip is operatively interconnected to the readout device108.

FIG. 5 illustrates a test strip 104 in accordance with other embodimentsof the present invention in plan view. The test strip 104 generallyincludes a substrate 132 with a test strip overlay 128 that covers atleast a portion of the substrate 132. The test strip overlay 128includes a test strip aperture 124 in an area corresponding to a samplechamber 120. As shown, a first area 212 of a test lead 208 extends intothe sample chamber 120. A second area 216 of the test lead 208corresponding to the readout contact 136 is on a portion of thesubstrate 132 that corresponds to the readout region 140 of the teststrip 104, and is not covered by the test strip overlay 128.

FIG. 6 is a cross-section of the test strip 104 illustrated in FIG. 5,taken along section line A-A. In this embodiment, the reference cell 224is contained within a gel volume 604. The gel volume 604 is defined byan aperture 608 formed in the substrate 132. The bottom of the gelvolume 604 is bounded by a reference cell carrier plate 612. The top ofthe gel volume 604 is partially closed by the test strip overlay 128.

FIG. 7 is a partial cross-section of the test strip 104 illustrated inFIG. 6, taken from within detail area B. As shown in FIG. 7, a notch 704in the aperture 608 formed in the substrate 132 at least partiallyoverlaps with the test strip aperture 124 formed in the test stripoverlay 128. Accordingly, the gel volume 604 is in communication withthe sample chamber 120. As a result, at least a portion of a fluidsample 116 placed in the sample chamber 120 can enter the gel volume604, such that the fluid sample 116 comes into contact with a gel 708.More particularly, the gel 708 at least partially fills the gel volume604. In accordance with embodiments of the present invention, the gel708 may comprise an ionic or electrolytic solution. Accordingly, the gel708 functions to place the fluid sample into electrical contact with thereference cell 224.

With reference again to FIG. 5, it can be seen that the notch 704 in theaperture 608 formed in the substrate 132 and the test strip aperture 124formed in the test strip overlay 128 cooperate to place the samplechamber 120 in communication with the gel volume 604.

Also visible in FIG. 7 is a filter 304 that covers the sample chamber120. The filter 304 can be a membrane that separates blood plasma fromwhole blood placed in or over the sample chamber 120, so that the bloodplasma comes into contact with the first area 212 of the test lead 208and the gel 708 in the gel volume 604. In this exemplary embodiment, thereference lead 220 is on a side of the substrate 132 opposite the sidethat carries the test lead 208. The reference lead 220 may be placedinto electrical contact with the reference cell 224 through electricalcontact with an electrically conductive carrier plate 612.

FIG. 8 is an exploded view of the test strip 104 illustrated in FIG. 5.In this exploded view, it can be seen that the working electrode 208 isformed on the substrate 132, and extends from the first area 212 to thesecond area 216. In addition, in this embodiment the reference cell 224is centered on an electrically conductive reference cell carrier plate612.

FIG. 9 is a top plan view of the substrate 132 of the test strip 104shown in FIG. 5, FIG. 10 is a bottom plan view of that test stripsubstrate 132, and FIG. 11 is a view of that test strip substrate 132 inelevation. As shown in FIG. 9, the aperture 608 in the substrate 132 maybe circular, with a notch 704 formed in a periphery thereof. FIG. 10shows the reference lead 220 that is formed on a side of the substrate132 opposite the side carrying the working lead 208. In particular, thereference lead 220 can include a circular portion that surrounds an areaoutside of the gel volume 604. Moreover, the test lead 208 and thereference lead 220 may be formed on opposite sides of the substrate 132(see FIG. 11).

With reference now to FIG. 12, an exploded view of a test strip 104 inaccordance with further embodiments of the present invention isillustrated. In particular, this embodiment includes a capsule 1204 thatcontains an ionic gel or other electrolyte. A wicking member 1208 isplaced under the capsule 1204. The wicking member 1208 includes a tab1212 that is in communication with the sample chamber 120. In use, thecapsule 1204 is broken, wetting the wicking member 1208 and therebyestablishing a salt bridge between the reference cell 224 and a samplefluid 116 in the sample chamber 120. In the assembled test strip 104,the gel capsule 1204 and the wicking member 1208 are held within anaperture 608 formed in the substrate 132, between the test strip overlay128 and the reference cell carrier plate 612.

FIG. 13 is a block diagram depicting components of a readout device 108in accordance with embodiments of the present invention. In general, thereadout device 108 includes a plurality of readout device contacts 144.The readout device contacts 144 may be associated with a receivingstructure, such as the aperture 148 illustrated in FIG. 1, formechanically interconnecting the readout device 108 to a test strip 104,to facilitate an electrical interconnection between at least two readoutdevice contacts 144 and at least two electrical contacts 136 of the teststrip 104. Alternatively or in addition, the readout device contacts 144may comprise conductive leads or probes that can be selectively placedinto contact with electrical contacts 136 of a test strip 104.

The readout device 108 also includes or comprises a voltmeter or readoutelectronics portion 1304. As can be appreciated by one skilled in theart, the readout electronics 1304 can be implemented in various ways.For example, the readout electronics 1304 may comprise a galvanostat. Asanother example, the endpoint electronics may comprise a potentiostat.As a further example, the readout electronics 1304 may comprise adigital voltmeter that includes an integrating converter. In accordancewith further embodiments, the readout electronics 1304 can comprise ananalog voltmeter or a digital or analog null balance voltmeter.

A processor 1308 that includes and/or is associated with memory 1312 canbe provided for controlling various aspects of the operation of thereadout device 108. The processor 1308, for example executinginstructions stored in memory 1312, can implement a process according towhich the voltage between the working electrode 208 (or alternativelythe counter electrode 236) and the reference electrode 220 is monitoredover time by the readout electronics 1304. Moreover, this voltage can bemonitored while the readout electronics 1304 applies a current across atleast the counter electrode 236 and the working electrode 208. Theprocessor 1308 can further operate to calculate and cause to bedisplayed a readout indicative of the oxidation-reduction potential of afluid sample 116 held in the sample chamber 120 from the voltage read bythe readout electronics 1304.

For providing information regarding the determined oxidation-reductionpotential of a fluid sample 116 in the sample chamber 120 to a user, auser output 152 is provided. The user output 152, can, in an exemplaryembodiment, comprise a digital output that displays anoxidation-reduction potential value. Alternatively or in addition, theuser output 152 can include indicator lamps, an analog output, or othervisually discernable output. In accordance with still furtherembodiments, the user output 152 can include an audible output, such asa selected tone or sequence of tones or machine-generated speech.

A user input 156 can be included for receiving control information froma user. For example, the user input 156 may receive input to power on orpower off the readout device 108, to perform diagnostics related to theproper operation of the readout device 108, to receive input regardingvarious operating parameters, or other user input. As examples, the userinput 156 can include buttons, switches, keypads, and/or a touch screeninterface integrated with a visual display, such as may be included inthe user output 152.

The readout device 108 may additionally include a communicationsinterface 1316. The communications interface 1316, if provided, maysupport interconnections between the readout device 108 and othersystems or devices. For example, the communications interface 1316 maycomprise a wired or wireless Ethernet connection, a universal serial busport, or an IEEE 1394 port for interconnecting the readout device 108 toa personal computer or computer network.

In addition, although an exemplary readout device 108 comprising adedicated standalone device that may or may not be interconnected toother devices has been described, embodiments of the present inventionare not so limited. For example, a readout device 108 in accordance withembodiments of the present inventions may be implemented as a standardvoltmeter. In accordance with other embodiments, the readout device 108may comprise an electrical test or diagnostic system, such as a userconfigurable potentiostat and/or galvanostat operated alone or incombination with a personal computer. In accordance with still otherembodiments, a readout device 108 may be implemented as a personalcomputer running suitable programming and providing an interface capableof sensing a voltage between a working electrode 208 and a referenceelectrode 220 of a test strip 104.

FIG. 14 illustrates aspects of a method for determining theoxidation-reduction potential of a fluid sample 116 in accordance withembodiments of the present invention. Initially, at step 1404, a fluidsample 116 is obtained from a test subject or patient. In accordancewith embodiments of the present invention, the fluid sample 116comprises whole blood or a blood product, such as plasma. As can beappreciated by one skilled in the art, a fluid sample 116 comprisingwhole blood or a blood product can be obtained from a test subject, forexample using a syringe and needle or a lancet. In accordance withfurther embodiments, the fluid sample can include any fluid from aliving test subject. Moreover, a test subject can include a human or anyother mammal or animal.

At step 1408, the fluid sample 116 is placed in the sample chamber 120of a test strip 104. Where the fluid sample 116 comprises plasma, theplasma may be separated from the whole blood in a separate process.Alternatively, where the sample fluid 116 comprises whole blood, afilter 304 over the sample chamber 120 may operate to filter othercomponents of the whole blood from a plasma component. The plasmacomponent of the fluid sample 116 is then allowed to collect in thesample chamber 120 or a portion of the sample chamber 120.

At step 1412, an electrically conductive bridge 232 between thereference cell 224 and the sample chamber 120 is established. Inaccordance with at least some embodiments of the present invention, thiscan be accomplished by wetting a bridge 232 formed using at least aportion of a filter 304 comprising a strip of filter paper, therebyestablishing a salt bridge connection between the sample chamber 120 andthe reference cell 224. In accordance with other embodiments, this canbe accomplished by placing the fluid sample 116 in contact with anelectrolytic gel that is also in contact with the reference cell 224,either directly or in connection with a filter 304 and/or a bridge 232.At step 1416, the test lead 208 and the reference lead 220 areinterconnected to the electrical contacts 144 of a readout device 108.At step 1420, the voltage or electrical potential between the workingelectrode or test lead 208 and the reference electrode 220 isdetermined. After a selected interval has elapsed, a subsequent readingof the voltage between the working electrode or test lead 208 and thereference cell electrode 220 is taken (step 1424). At step 1428, adetermination is made as to whether the rate of change between the tworeadings indicates that the system has reached equilibrium and thereforethat a reliable reading has been obtained. If it is determined that thesystem has not reached equilibrium, the system returns to step 1424, anda further subsequent reading of the voltage between the workingelectrode 208 and the reference cell electrode 220 is taken. If it isdetermined at step 1428 that the system has stabilized, the measure ofthe oxidation-reduction potential of the fluid sample 116 in the samplechamber 120 can be output (step 1432). For example, an indication of theoxidation-reduction potential of the fluid sample 116 can be outputthrough the user output 152 and/or output to another device through acommunications interface 1316.

In accordance with still other embodiments, a curve fitting proceduremay be performed in order to determine the oxidation-reduction potentialof the sample 116. For example, the voltage between the workingelectrode 208 and the reference cell electrode 220 can be taken at atleast three different points in time, and the data thus obtained can beapplied to a curve fitting algorithm to arrive at an oxidation-reductionpotential reading. The curve fitting algorithm may comprise a diffusionequation, a polynomial curve fitting algorithm, or any other curvefitting algorithm.

In accordance with embodiments of the present invention, a test strip104 may be formed using a substrate 132 that comprises any dielectricmaterial capable of providing mechanical support to the leads 204 andother components. Accordingly, the substrate 132 may comprise plastic,ceramic, glass, or other material. Moreover, the substrate 132 maycomprise a planar sheet of material. The leads 204 may be formed throughvarious means. For example, the leads 204 may be deposited as aconductive ink on the substrate 132. Examples of suitable conductive inkinclude graphite inks and noble metals, such as gold, platinum oriridium. Leads 204 may also be formed through various other depositionand/or etching processes. Moreover, the reference cell 224 and bridge232 may be applied by placing appropriate materials on the substrate132.

The test strip overlay 128 may comprise the same or a similar materialas the substrate 132. Moreover, the test strip overlay 128 can include atest strip aperture 124 corresponding to the sample chamber 120. Thetest strip overlay 128 may be bonded to the substrate 132, such thatsome or all of the other components, such as the leads 204, referencecell 224 and bridge 232, are at least partially held between asubstantially planar top surface of the substrate 132 and asubstantially planar bottom surface of the test strip overlay 128.

The reference cell 224 may comprise any chemical half cell or electrodethat is capable of providing a known reference voltage. Accordingly, thereference cell 224 may comprise a standard hydrogen electrode, asilver/silver chloride electrode, a calomel electrode, a mercuroussulfate electrode, a mercuric oxide electrode, or a copper/coppersulfate electrode. In embodiments of a test strip 104 incorporating agel 708, that gel 708 may comprise any ionic liquid, electrolyticsolution or ionic gel. Examples of suitable gels 708 include cationicpolymers, ionic liquids, and gelled electrolytes.

A further embodiment of the present invention is now described withreference to FIGS. 15 and 16. FIG. 15 illustrates an exploded view of atest strip 104 in accordance with embodiments of the present invention.FIG. 16 illustrates the test strip 104 of FIG. 15 in the top plan view.The test strip 104 includes a substrate 132. More particularly, thesubstrate 132 in this exemplary embodiment includes a structural supportlayer 1504 and a barrier layer 1508. The barrier layer 1508 may comprisea layer that is impermeable to liquids. For example, the barrier layer1508 may comprise an oriented polyester film, such as but not limitedto, a biaxially-oriented polyethylene terephthalate, such as Mylar™. Thestructural support layer 1504 may comprise a fiber or polymer layer thatis sufficiently rigid to provide mechanical support for the subsequentlayers, such as but not limited to a polyester material.

Electrically conductive leads 204 are supported by the barrier layer1508. As an example, and without limitation, the conductive leads 204can be deposited on the surface of the barrier layer 1508 by asputtering, printing, etching, stenciling, or plating process. Theelectrically conductive leads 204 may be formed from any electricallyconductive material. Examples of suitable electrically conductivematerials include platinum, gold and doped carbon. The conductive leads204 can be formed in various patterns. In general, the conductive leads204 include a working electrode 208, a reference electrode 220 and acounter electrode 236.

A reference cell 224 can be placed within a gel 708 deposited on thebarrier layer 1508. Moreover, at least some of the gel 708 is placedover or in contact with a portion of the reference lead or electrode220. A dielectric layer 1512 may be placed over or formed on portions ofthe barrier layer 1508. For example, the dielectric layer 1512 can coverportions of the various electrically conductive leads 204, while leavingportions of the electrically conductive leads 204 corresponding to areadout region 140 of the electrically conductive leads 204 uncovered.In addition, the dielectric layer 1512 can include a first aperture 1516that leaves a first area 212 of the working electrode 208 and a firstarea 240 of the counter electrode 236 uncovered and exposed to a volumecorresponding to a sample chamber 120. The dielectric layer 1512 canadditionally include a second aperture 1520. The second aperture 1520can correspond to the reference cell 224 and/or the gel 708. As anexample, the dielectric layer 1512 may be formed from a dielectric film,or a deposited (e.g., a printed) dielectric material.

A filter 304 is provided that extends from an area encompassing at leastpart of the first aperture 1516 and the second aperture 1520 of thedielectric layer 1512. As with other embodiments described herein, thefilter 304 can function, when wetted, as a bridge 232 to electricallyconnect a portion of a sample 116 within the sample chamber 120 to thereference cell 224, directly or through the gel 708.

A spacer layer 1524 is interconnected to the dielectric layer 1512. Thespacer layer 1524 includes a spacer layer aperture 1528. The spacerlayer aperture 1528 may have an area that is the same as or larger thanan area of the filter 304. Accordingly, the spacer layer aperture 1528can define the perimeter of a volume that is entirely or substantiallyoccupied by the filter 304.

Next, a test strip overlay 128 can be interconnected to the spacer layer1524. The test strip overlay 128 generally includes an overlay aperture124. In general, the overlay aperture 124 cooperates with the spacerlayer 1524 aperture 1528 to define portions of a sample chamber 120.

In accordance with embodiments of the present disclosure, the structuralsupport layer 1504 and the barrier layer 1508 have the same orsubstantially similar lengths and widths, and are adhered or bonded toone another to form the laminated substrate 132. The dielectric layer1512, spacer layer 1524, and test strip overlay layer 128 have the sameor a similar length and width as one another, and a length that is lessthan the length of the laminated substrate 132. Accordingly, thedielectric layer 1512, spacer layer 1524, and test strip overlay layer128 leave a readout region 140 of the test strip 104 electricallyconductive leads 204 uncovered.

A test strip 104 in accordance with embodiments of the present inventioncan additionally include a protective layer 1532. The protective layer1532 may have a length and width that is the same or similar to thelength and width of the substrate 132, to cover the top surface of thetest strip 104 (i.e., the surface of the test strip 104 opposite thesubstrate 132) in its entirety. Accordingly, the protective layer 1532is removed from the test strip 104 before use. The protective layer 1532can comprise a sealer film, such as a polymeric material.

At least one of the leads 204 is a working electrode or first test lead208 that extends between a first area 212 corresponding to or within thesample chamber 120 and a second area 216 corresponding to the readoutcontact 136 of the working electrode 208. Another lead 204 comprises areference lead 220. The reference lead 220 extends between the referencecell 224 and a readout region of the reference lead 228. Moreover, inaccordance with embodiments of the present invention the test strip 104may optionally include a second test lead or counter electrode 236. Thecounter electrode 236 may generally mirror the working electrode 208.

In accordance with embodiments of the present invention, at least some,if not most or all, of the leads 204 are formed by printing anelectrically conductive material. Non-limiting examples of electricallyconductive materials are carbon (such as carbon black, carbon nanotubes,graphene sheets, graphite and bucky balls), metallic materials (such aspowder forms of copper, silver, gold and other known conductive metallicmaterials) and conductive polymers. Furthermore, the conductive materialis printed in the form of a substantially continuous and/or uniformcomposition, as described above. In accordance with further embodiments,the leads 204 are formed by sputtering gold, platinum, or some othermetal.

A top plan view of the test strip 104 illustrated in FIG. 15 is shown inFIG. 16. In this view, the sealer film 1532, test strip overlay 128,spacer 1524, filter 304, and dielectric layer 1512 are depicted as beingtransparent, so that the relative positions of various components of thetest strip 104 can be seen.

The test strip 104 forms an electrochemical test cell. In particular,when a blood sample has been placed in the sample cell 120, for examplethrough the test strip overlay layer 128 aperture 124, theelectrochemical test cell comprises the separated plasma, containedwithin the sample chamber 120 and wetting the filter 304, the gel 708,and the reference cell 224. The electrical potential of the test cellcan then be read by interconnecting at least one of the workingelectrode 208 and counter electrode 236, and the reference lead 220 to areadout apparatus or device 108.

FIG. 17 illustrates an example of a test strip 104 in accordance withother embodiments of the present invention in top plan view. In thisexample, the sealer film 1532, test strip overlay 128, spacer 1524,filter 304, and dielectric layer 1572 are depicted as being transparent,so that the relative positions of various components of the test strip104 can be seen. The filter 304 extends from the sample chamber 120 toan area including a gel 708. The reference cell 224 in this embodimentcomprises a Ag/AgCl half cell that is surrounded by a hydroxyethylcellulose gel 708. In addition, at least a portion of the reference cell224 may be in direct contact with the reference lead 220. Theelectrically conductive leads 204 can comprise sputtered gold and/orsputtered platinum. The use of sputtered metal can provide a moreuniform surface than a conductive ink. Alternatively, the electricallyconductive leads 204 can be formed from electrically conductive ink. Asan example, the electrically conductive leads 204 can be deposited in alayer that is about 5,000 Angstroms thick.

In accordance with embodiments of the present invention, the procedurefor applying the gel 708 over the reference cell 224 can be controlled,in order to obtain more consistent results. For example, the gel 708 canbe dried under conditions that limit or reduce the formation ofmicrocracks or other discontinuities. Accordingly, drying the gel 708can be performed at ambient temperatures and pressures, while applyingheat, in a vacuum, and the like. As an alternative to a dried gel 708, agel 708 can be contained within a capsule, which is broken immediatelyprior to use of the test strip 104. Alternatively or in addition,different gel 708 compositions can be used. For example, a gelcomprising a hydroxyethyl cellulose material can be mixed with a polymerto promote consistency of the gel 708 in finished test strips 104.

FIG. 18 depicts components of a readout device 108 operativelyinterconnected to a test strip 104 in accordance with embodiments of thepresent invention. More particularly, features of a voltmeter or readoutelectronics portion 1304 of a readout device 108 interconnected to atest strip 104 containing a fluid sample 116 are depicted. As can beappreciated by one of skill in the art after consideration of thepresent disclosure, the test strip 104 containing a fluid sample 116comprises an electrochemical cell 1828. The electrochemical cell 1828includes the fluid sample 116, the electrolytic gel 708 (if provided),and the reference cell 224. Moreover, the fluid sample 116, for exampleby wetting a bridge 232 and/or filter 304, places portions of theworking electrode 208, reference electrode 220, and counter electrode236 in electrical contact with one another.

In general, the readout electronics 1304 include a power amplifier 1804.The output 1808 from the power amplifier 1804 comprises a current havinga set point determined by the voltage V_(set) 1812 provided as an inputto the power amplifier 1804. The output current 1808 from the poweramplifier 1804 is passed to a current-potential (IE) converter 1816. Thecurrent 1808 from the power amplifier 1804 can be supplied via aresistor 1820 to the negative input of the IE converter 1816. The IEconverter 1816 in turn supplies an output current 1824 that is providedto the counter electrode 236. The negative input of the IE converter1816 is additionally connected to the working electrode 208. As can beappreciated by one of skill in the art after consideration of thepresent disclosure, the resistance between the counter electrode 236 andthe working electrode 208 can vary, depending on the composition andcharacteristics of a fluid sample 216 placed in the test strip 104.However, the power amplifier 1804 and the IE converter 1816, incombination, provide a constant current that is supplied to the counterelectrode 236, and that is passed through the electrochemical cell 1828.

While the current is applied across the counter electrode 236 and theworking electrode 208, the voltage potential between the workingelectrode 208 and the reference electrode 220 is monitored by adifferential amplifier or electrometer 1832. More particularly, thedifferential amplifier 1832 provides a voltage output 1836 that isindicative of the oxidation-reduction potential of the sample 116 placedwithin the sample chamber 120. This voltage output 1836 can be presentedto a user, for example through the output 152 of the associated readoutdevice 108.

With reference now to FIG. 19, aspects of a method for measuring theoxidation-reduction potential (ORP) of a sample fluid 116 areillustrated. In general, the method includes steps of obtaining a fluidsample 116 (step 1904), placing the fluid sample 116 in the samplechamber 120 of a test strip 104 (step 1908), and establishing anelectrically conductive bridge 232 between the reference cell 224 andthe sample chamber 120 of the test strip 104, for example by wetting afilter 304 with the sample fluid 116 (step 1912). Accordingly, steps1904 to 1912 are the same or similar as steps 1404 to 1412 described inconnection with FIG. 14 above.

At step 1916, the working electrode 208, reference electrode 220, andcounter electrode 236 are interconnected to readout device contacts 144.For example, the counter electrode 236 can be interconnected to thecurrent output 1824 of the readout electronics 1304, the workingelectrode 208 can be connected to the negative inputs of the IEconverter 1816 and the differential electrometer 1836 of the readoutelectronics 1304 and the reference electrode 220 can be interconnectedto an input of the differential amplifier 1832. The readout electronics1304 are then operated to provide a current that is passed across thereference cell 1828, between the counter electrode 236 and the workingelectrode 208 (step 1920). As examples, and without limitation, theamount of current passed between the counter electrode 236 and theworking electrode 208 by the readout electronics 1304 can be from about10⁻¹² amps to about 10⁻⁹ amps. In accordance with further embodiments,the magnitude of the current passed through the electrochemical cell1828 can be from about 1×10⁻¹⁴ amps to about 1×10⁻⁶ amps. As furtherexamples, the applied current can be varied over time. For instance, astep function can be followed, according to which the applied currentchanges after some point of time from a first value (e.g., 10⁻⁹ amps) toa second value (e.g., 10⁻¹¹ amps). While the current is applied betweenthe counter electrode 236 and the working electrode 208, the potentialdifference between the working electrode 208 and the reference electrode220 is provided as the output 1836 of the differential amplifier 1832(step 1924).

The output 1836 from the differential amplifier 1832 can be monitoredover time (step 1928). At step 1932, a determination can be made as towhether equilibrium has been reached. The determination that equilibriumhas been reached can include monitoring the rate of change in the outputsignal 1836 of the differential amplifier 1832, until that rate ofchange has dropped to a predetermined level. Alternatively, the outputvoltage 1836 can be measured at different points in time, and a linearor curved representation of the change in the voltage output 1836 can beused to arrive at an oxidation-reduction potential reading. Ifequilibrium has been reached, the determined oxidation-reductionpotential value is presented to a user of the readout device 108 (step1936). For example, the determined oxidation-reduction potential valuecan be presented as a measured voltage. If equilibrium has not beenreached, the process can return to step 1920. After the ORP value hasbeen output, the process can end.

FIG. 20A is a graph depicting exemplary ORP values for normal plasma asread using a number of sample test strips in accordance with embodimentsof the present invention. FIG. 20B is a graph depicting exemplary ORPvalues for trauma plasma using a number of different sample test strips.The test strip 104 used to obtain the ORP values is configured like theexemplary test strip 104 illustrated in FIG. 17. In addition, each teststrip 104 incorporated a 10 μL 4% Agarose/3M KCl gel 708, a small saltbridge 232, a center spot comprising the reference cell 224, andsputtered platinum electrically conductive leads 204. The ORP valueswere read using readout electronics 1302 comprising a galvanostat, asgenerally shown in FIG. 18. The readout current applied by the readoutelectronics 1302 was 1×10⁻⁹ amps. As shown in the figures, the potential(the vertical axis in the graphs) diminishes over time (the horizontalaxis). In addition, a comparison of FIGS. 20A and 20B reveals that theORP value, as expressed by the measured potential in millivolts, ishigher for the trauma plasma (i.e., the plasma taken from an animal whohas suffered a trauma) as compared to the measured ORP value for plasmafrom a normal patient. More particularly, after three minutes, themeasured ORP of the trauma plasma was an average of 218.3 mV±6.4, whilethe average ORP for the normal plasma was 171.6 mV±3.6. In accordancewith embodiments of the present invention, the ORP value used fordiagnostic purposes would be the value arrived at after sufficient timehas elapsed for the ORP to have settled such that the rate of change inmeasured ORP values is less than some selected amount. Alternatively orin addition, a curve fitting procedure can be used to extrapolate to anORP value reported to the clinician or other user as a measured orderived ORP value.

The foregoing discussion of the invention has been presented forpurposes of illustration and description. Further, the description isnot intended to limit the invention to the form disclosed herein.Consequently, variations and modifications commensurate with the aboveteachings, within the skill or knowledge of the relevant art, are withinthe scope of the present invention. The embodiments describedhereinabove are further intended to explain the best mode presentlyknown of practicing the invention and to enable others skilled in theart to utilize the invention in such or in other embodiments and withvarious modifications required by the particular application or use ofthe invention. It is intended that the appended claims be construed toinclude alternative embodiments to the extent permitted by the priorart.

1. An oxidation-reduction potential test device, comprising: asubstrate; a sample chamber; a first test lead supported by thesubstrate, the first test lead including: a first area extending intothe sample chamber; a second area extending from the sample chamber; areference cell; a reference lead, including; a first area in electricalcontact with the reference cell; a second area extending from thereference cell; a bridge, wherein a fluid sample in the sample chamberis placed in electrical contact with the reference cell by the bridge;an overlay, wherein at least a portion of the first test lead, thereference cell, at least a portion of the reference lead, and the bridgeare held between the substrate and the overlay when the overlay isinterconnected to the substrate, wherein the overlay includes anaperture, and wherein the aperture corresponds to at least a portion ofthe sample chamber when the overlay is interconnected to the substrate;a filter element, wherein the filter element at least one of extendsover and is contained within the sample chamber.
 2. The device of claim1, further comprising: a gel volume, wherein the bridge comprises anelectrolytic gel contained within the gel volume, and wherein theelectrolytic gel is in contact with the reference cell.
 3. The device ofclaim 2, wherein the gel volume is in communication with the samplechamber.
 4. The device of claim 1, wherein the reference cell includes asilver/silver chloride reference cell.
 5. The device of claim 1, whereina first substance added to the bridge comprises a wetting agent, andwherein the bridge and the wetting agent form a salt bridge electricallyinterconnecting the first area of the first test lead to the referencecell.
 6. The device of claim 1, wherein the first test lead is formedfrom an electrically conductive material having a constant composition.7. The device of claim 1, wherein the first test lead includes a readout portion, wherein the reference lead includes a readout portion,wherein the read out portion of the first test lead and the readoutportion of the reference lead are accessible to a test device when theoverlay is interconnected to the substrate.
 8. A method for determiningthe oxidation-reduction potential of a fluid sample, comprising:providing a test strip with a sample chamber, a reference cell, a firsttest lead, and a reference lead; placing a fluid sample in a samplechamber, wherein the first test lead and the reference lead areelectrically interconnected to one another via the fluid sample in thesample chamber and a salt bridge between the sample chamber and thereference cell; reading a voltage from the sample chamber via the firsttest lead and the reference lead, wherein the reference lead isinterconnected to the sample chamber by the reference cell and thebridge.
 9. The method of claim 8, further comprising: providing a firstfilter element extending between the sample chamber and the referencecell, wherein the bridge is formed from wetting of the filter element bya component of the fluid sample.
 10. The method of claim 9, furthercomprising: providing a second filter element overlaying the samplechamber, wherein placing the fluid sample in the sample chamber includesfiltering the fluid using the second filter element.
 11. The method ofclaim 9, wherein the first filter element overlays the sample chamber,and wherein placing the fluid sample in the sample chamber includesfiltering the fluid using the first filter element.
 12. The method ofclaim 8, further comprising: providing a gel volume that is incommunication with the sample chamber, wherein a gel within the gelvolume electrically interconnects the fluid sample or a component of thefluid sample to the reference cell.
 13. The method of claim 8, whereinproviding a test strip with a sample chamber, a reference cell, a firsttest lead, and a reference cell further comprises: providing asubstrate; providing an overlay; providing the first test lead, whereinthe first test lead has a sample portion, a transfer portion, and areadout portion, and wherein the first test lead is a substantiallyuniform, electrically conductive material; providing a bridgingmaterial; providing the reference cell; providing the reference leadhaving a reference cell contact portion electrically interconnected tothe reference cell, and a readout portion; interconnecting the overlayto the substrate, wherein the test lead, the bridging material, and thereference cell are all at least partially held between the overlay andthe substrate, wherein one of the substrate and the overlay includes asample chamber aperture defining a sample chamber, wherein the sampleportion of the test lead extends into the sample chamber, and whereinthe readout portion of the test lead and a readout portion of thereference lead are accessible to a readout device.
 14. The method ofclaim 8, wherein the fluid sample is whole blood.